【发布时间】：2016-05-22 21:01:11
【问题描述】：
我正在尝试进行 C 编程，但在从 C 数组分配和提取数据时遇到问题（在本例中，是与 C 风格的字符串之间）。
请指出您在此处看到的任何错误。
我主要是一名c++/python程序员，所以请尽量简单地解释内存使用和管理。
#include <stdio.h>
typedef struct AuthorInfo {
char* firstName;
char* lastName;
} AuthorInfo;
typedef struct BookEntry {
char bookID;
char* bookName;
AuthorInfo author;
} BookEntry;
void assign_str(const char** from, char** to) {
int size = sizeof(from)/sizeof(char);
printf((char)size);
printf('\n');
for (int i=0; i < size; i++) {
(*to)[i] = (*from)[i];
};
};
BookEntry BookEntry_(const int id, const char* bName, const char* aF, const char*aL, BookEntry* ret) {
ret->bookID = id;
ret->bookName = (char*)malloc(sizeof(bName));
ret->author.firstName = (char*)malloc(sizeof(aF));
ret->author.lastName = (char*)malloc(sizeof(aL));
assign_str(bName, &ret->bookName);
assign_str(aF, &ret->author.firstName);
assign_str(aL, &ret->author.lastName);
}
void display_book(BookEntry* entry) {
printf(entry->bookName);
printf('\n');
printf(entry->author.firstName);
printf(' ');
printf(entry->author.lastName);
printf('\n');
};
int main(int argc, char** args) {
BookEntry book;
book.bookID = 0;
assign_str("Tom Sawyer", &book.bookName);
assign_str("Mark", &book.author.firstName);
assign_str("Twain", &book.author.lastName);
display_book(&book);
return 0;
};
使用gcc goof.c -o goof -std=c11 编译此代码会导致：
goof.c: In function ‘assign_str’:
goof.c:16:10: warning: passing argument 1 of ‘printf’ makes pointer from integer without a cast [-Wint-conversion]
printf((char)size);
^
In file included from goof.c:1:0:
/usr/include/stdio.h:362:12: note: expected ‘const char * restrict’ but argument is of type ‘char’
extern int printf (const char *__restrict __format, ...);
^
goof.c:16:3: warning: format not a string literal and no format arguments [-Wformat-security]
printf((char)size);
^
goof.c:17:10: warning: passing argument 1 of ‘printf’ makes pointer from integer without a cast [-Wint-conversion]
printf('\n');
^
In file included from goof.c:1:0:
/usr/include/stdio.h:362:12: note: expected ‘const char * restrict’ but argument is of type ‘int’
extern int printf (const char *__restrict __format, ...);
^
goof.c:17:3: warning: format not a string literal and no format arguments [-Wformat-security]
printf('\n');
^
goof.c: In function ‘BookEntry_’:
goof.c:25:26: warning: implicit declaration of function ‘malloc’ [-Wimplicit-function-declaration]
ret->bookName = (char*)malloc(sizeof(bName));
^
goof.c:25:26: warning: incompatible implicit declaration of built-in function ‘malloc’
goof.c:25:26: note: include ‘<stdlib.h>’ or provide a declaration of ‘malloc’
goof.c:28:14: warning: passing argument 1 of ‘assign_str’ from incompatible pointer type [-Wincompatible-pointer-types]
assign_str(bName, &ret->bookName);
^
goof.c:14:6: note: expected ‘const char **’ but argument is of type ‘const char *’
void assign_str(const char** from, char** to) {
^
goof.c:29:14: warning: passing argument 1 of ‘assign_str’ from incompatible pointer type [-Wincompatible-pointer-types]
assign_str(aF, &ret->author.firstName);
^
goof.c:14:6: note: expected ‘const char **’ but argument is of type ‘const char *’
void assign_str(const char** from, char** to) {
^
goof.c:30:14: warning: passing argument 1 of ‘assign_str’ from incompatible pointer type [-Wincompatible-pointer-types]
assign_str(aL, &ret->author.lastName);
^
goof.c:14:6: note: expected ‘const char **’ but argument is of type ‘const char *’
void assign_str(const char** from, char** to) {
^
goof.c: In function ‘display_book’:
goof.c:34:3: warning: format not a string literal and no format arguments [-Wformat-security]
printf(entry->bookName);
^
goof.c:35:10: warning: passing argument 1 of ‘printf’ makes pointer from integer without a cast [-Wint-conversion]
printf('\n');
^
In file included from goof.c:1:0:
/usr/include/stdio.h:362:12: note: expected ‘const char * restrict’ but argument is of type ‘int’
extern int printf (const char *__restrict __format, ...);
^
goof.c:35:3: warning: format not a string literal and no format arguments [-Wformat-security]
printf('\n');
^
goof.c:36:3: warning: format not a string literal and no format arguments [-Wformat-security]
printf(entry->author.firstName);
^
goof.c:37:10: warning: passing argument 1 of ‘printf’ makes pointer from integer without a cast [-Wint-conversion]
printf(' ');
^
In file included from goof.c:1:0:
/usr/include/stdio.h:362:12: note: expected ‘const char * restrict’ but argument is of type ‘int’
extern int printf (const char *__restrict __format, ...);
^
goof.c:37:3: warning: format not a string literal and no format arguments [-Wformat-security]
printf(' ');
^
goof.c:38:3: warning: format not a string literal and no format arguments [-Wformat-security]
printf(entry->author.lastName);
^
goof.c:39:10: warning: passing argument 1 of ‘printf’ makes pointer from integer without a cast [-Wint-conversion]
printf('\n');
^
In file included from goof.c:1:0:
/usr/include/stdio.h:362:12: note: expected ‘const char * restrict’ but argument is of type ‘int’
extern int printf (const char *__restrict __format, ...);
^
goof.c:39:3: warning: format not a string literal and no format arguments [-Wformat-security]
printf('\n');
^
goof.c: In function ‘main’:
goof.c:45:14: warning: passing argument 1 of ‘assign_str’ from incompatible pointer type [-Wincompatible-pointer-types]
assign_str("Tom Sawyer", &book.bookName);
^
goof.c:14:6: note: expected ‘const char **’ but argument is of type ‘char *’
void assign_str(const char** from, char** to) {
^
goof.c:46:14: warning: passing argument 1 of ‘assign_str’ from incompatible pointer type [-Wincompatible-pointer-types]
assign_str("Mark", &book.author.firstName);
^
goof.c:14:6: note: expected ‘const char **’ but argument is of type ‘char *’
void assign_str(const char** from, char** to) {
^
goof.c:47:14: warning: passing argument 1 of ‘assign_str’ from incompatible pointer type [-Wincompatible-pointer-types]
assign_str("Twain", &book.author.lastName);
^
goof.c:14:6: note: expected ‘const char **’ but argument is of type ‘char *’
void assign_str(const char** from, char** to) {
^
运行代码会导致 bash 说：
Segmentation fault (core dumped)
【问题讨论】：
-
添加一项重要内容：
#include <stdlib.h>。请务必阅读printf的文档。它适用于格式字符串，例如以这种方式打印数字：printf("Value: %d\n", val);。有一些格式可以修补字符串参数、单个字符、数字…… -
int size = sizeof(from)/sizeof(char);不会像你想要的那样使用函数参数。第一个sizeof将给出指针的大小，这是函数唯一知道的关于from的信息。 -
您对
printf的使用完全错误，请RTM。 -
而且
sizeof(char) != 1是不可能的。 -
错误消息会准确地告诉您您做错了什么。如果你想编程，一般来说，你需要结合阅读书籍和/或在线文档来解决问题并了解你所使用的语言。